1. Field of the Invention
The present invention relates to a shank of a very small sized rotating and cutting tool such as a drill appropriate for use in making small precise holes in printed boards, buttons or other small articles.
2. Description of the Prior Art
A variety of very small sized rotating and cutting tools have been widely used. Such a very small sized rotating and cutting tool is represented by a miniature drill, and it comprises a rotating and cutting part ranging from 0.3 mm to 1.5 mm in diameter and a shank integrally connected to the rotating and cutting part and having a diameter of 3.175 mm. This is hereinafter called a "small-and-large cylindrical object".
In making such a "small-and-large cylindrical object" a rod of a super hard alloy, such as WC-Co is prepared, and the rod is centerless-supported, and is subjected to surface grinding and subsequent end-cutting. Thereafter, the rod is machined into a "small-and-large cylindrical object". Specifically, a part of the rod which is to be a rotating and cutting part such as a drill bit, is subjected to coarse machining to reduce the diameter of the part to 0.3 to 1.5 mm, and then, the tip of the part is subjected to fine machining to be reduced into a tapered shape.
This machining requires strict precision as is attained for example, by an NC centerless cutting machine. The exact center alignment of the bit and shank is required and the bit size is very small (0.3 to 1.5 mm). The part of the rod to be reduced into a bit is fragile, and can be easily broken while being machined. Such precision machining is a factor for increasing the manufacturing cost.
After finishing such precision machining, the semimanufactured object is subjected to bit grooving, bit tip machining, undercutting and other necessary machining. Finally, a miniature drill results.
According to another conventional manufacturing process, a relatively large cylinder for a shank and a relatively small cylinder for a bit are prepared separately, and a hole for attaching the bit to the shank is made on one end of the relatively large cylinder. The relatively small cylinder is inserted into the hole of the relatively large cylinder, and is soldered to the relatively large cylinder. Thus, a miniature drill results.
This second manufacturing method is advantageous over the first manufacturing method because no precision machining using an NC centerless cutting machine is required and because machining is much easier than that in the first manufacturing method.
Miniature drills have a drill bit of very small diameter, resulting in a high likelihood that the drill bit will be broken while making holes. Also, the drill bit is likely to be worn. As for the miniature drill which is manufactured according to the first manufacturing method and is composed of a bit-and-shank integration; when its bit is worn or broken, the tool must be changed as a whole for a new one. This means that the shank of the tool is thrown away in spite of having no defect. Stated otherwise, the shank cannot be reused. This is economically disadvantageous, when the manufacturing cost of the bit-and-shank integration is considered.
As for the miniature drill which is manufactured according to the second manufacturing method, and is composed of an integral connection of separate bit and shank portions: when its bit is worn or broken, the tool must be changed as a whole for a new one, as is the case with the first miniature drill. It is most likely that the drill bit will be broken in the vicinity of the end of the shank, leaving the broken part in the hole of the shank. Therefore, the shank cannot be reused because it is actually impossible to remove the broken part from the hole of the shank.